Titanium dioxide photocatalytic compositions and uses thereof

ABSTRACT

Provided is a photocatalytic composition comprising zinc (Zn) doped titanium dioxide (TiO 2 ) nanoparticles, wherein the ratio of titanium dioxide nanoparticles to zinc is from about 5 to about 150. The photocatalytic composition absorbs electromagnetic radiation in a wavelength range from about 200 nm to about 500 nm, and the absorbance of light of wavelengths longer than about 450 nm is less than 50% the absorbance of light of wavelengths shorter than about 350 nm. Further provided is a method for treating or preventing microbial diseases and infestations in a plant and a method for increasing crop yield of a plant by applying the photocatalytic compositions taught herein to the surface of a plant. Also provided is a method for treating microbial diseases on a surface by applying the photocatalytic compositions taught herein to a surface illuminated by artificial light.

This application claims the benefit of U.S. Provisional Application No.61/482,393, filed May 4, 2011.

FIELD

The present disclosure relates to novel photocatalytic compositionscomprising titanium dioxide (TiO₂) nanoparticles, which are useful inthe treatment of microbial diseases, more specifically, microbialdiseases in plants.

BACKGROUND

In the past several decades, the development and exploration of theproperties of materials led to the recognition of the photocatalyticnature of crystalline metal oxides such as TiO₂ (Fujishima et al.,Nature, vol. 238, pgs. 37-38, 1972). Much effort has been devoted toresearch in this area resulting in a wide range of potentialapplications, such as sensors, photocatalysts, and photovoltaics. Theproperties of such materials depend on their chemical composition, size,and shape. In particular, as the particle size of the materialsdecreases, new physical and chemical properties may emerge as a resultof the greatly increased surface area. However, the relationship betweenphysical properties and the photocatalytic activities is complex, andoptimal conditions and structures may vary from case to case, asdiscussed in Chen et al. extensive review of the methods of synthesisand the physicochemistry of TiO₂ nanoparticles (Chemical Reviews, vol.107, pgs. 2891-2959, 2007).

Some years after the discovery of photocatalysis by TiO₂, studies showedthat TiO₂ acts as a light-activated antimicrobial coating whenirradiated for 60-120 minutes with ultraviolet (UV) radiation (387 nm);the coating was shown to have high bactericidal action againstEscherichia coli and Lactobacillus acidophilus (Matsunaga et al., FEMSMicrobiology Letters, vol. 29, pgs. 211-214, 1985). Subsequent work ledto development of nanoscale TiO₂ formulations that can have inhibitoryeffects on a range of bacterial, fungal and viral organisms (forexample, Tsuang et al., Artificial Organs, vol. 32, pgs. 167-174, 2008and Choi et al., Angle Orthodontist, vol. 79, pgs. 528-532, 2009)including organisms that increase the risk of hospital acquiredinfection when present on surfaces (Dancer, S. J., Lancet InfectiousDiseases, vol. 8, pgs. 101-113, 2008). Thus, when reduced microbialcontamination on inanimate surfaces is desired, nanoscale TiO₂ coatingscan be applied to that surface followed by UV illumination.

More recently, a few reports have emerged indicating that TiO₂ can beapplied to plants to provide certain benefits. Kawai proposed that thephotocatalyzed oxidative effect from application of a TiO₂ preparationdegrades organic material and thereby increases local CO₂ concentrationsat the leaf surface leading to increased plant sugar content, and alsocreates an antibacterial condition in at least some plants by theoxidation of plant lipids to induce endogenous plant defense mechanismsthat reduce the impact of pathogenic microbes (U.S. Pat. No. 6,589,912).A commercial photocatalytic nanoscale TiO₂ with an average particle sizeof 30 nm was reported to accelerate blooming and fruiting and reduce theincidence of certain diseases (Japanese Patent No. 2006-632721). Anothergroup also reported that TiO₂ particles averaging 30 nm reduced theextent of disease from two bacteria in cucumber leaves and alsoincreased the photosynthetic rate (Zhang et al., Nanoscience, vol.12(1), pgs. 1-6, 2007; Zhang et al., Journal of Inorganic Materials,vol. 23(1), pgs. 55-60, 2008; and Cui et al., NSTI-Nanotech, vol. 2,pgs. 286-289, 2009).

Nanoscale TiO₂ absorbs light in the UV range, but has very littleabsorbance in the visible range; this characteristic makes it a usefulcomponent in applications where protection from UV damage is helpful.However, in some applications it would be preferable to achieve thephotocatalytic effect with longer wavelength light. For example,interior lighting generally exhibits minimal UV energy, greatly reducingthe ability of nanoscale TiO₂ to exhibit photocatalysis. Similarly,greater photocatalytic efficiency in agricultural applications canreduce application rates and costs, and multiple benefits can beobtained by increasing the fraction of available solar irradiancecaptured by the photocatalyst. Thus, increasing the absorbance of longerwavelengths would allow the benefits of photocatalytic effects across awider range of applications.

Investigation over many years has shown that the absorption spectrum ofTiO₂ can be altered by introduction of doping agents that change thecrystal lattice structure. A more recent report shows that theabsorption spectrum can extend across the entire visible range toproduce a material that is black to the human eye (Chen et al., ScienceXpress, pgs. 1-10, online publication Jan. 20, 2011, Science. 1200448).However, such a broad absorbance spectrum is undesirable for use onplants, which are dependent on solar irradiation for photosynthesis.

The photosynthetic efficiency of plants varies across theelectromagnetic spectrum. The number of photons of a given energy orwavelength that are needed to give a certain photosynthetic rate can bemeasured, and when this is determined across a range of wavelengths oneobtains an action spectrum. Detailed action spectra have been reportedover a wide range of monochromatic light for various plant species. Asystematic study of the action spectra for 33 species of higher plantswas reported (Inada, K., Plant and Cell Physiology, vol. 17, pgs.355-365, 1976). Of interest is the observation that the action spectrafor all herbaceous plants is generally similar, with a high and broadpeak at 500-680 nm, which extends to a lower and narrower shoulder atabout 435 nm, with a rapid decline at shorter wavelengths. The spectrumfor arboreal plants is similar although the size of the 435 nm shoulderis reduced compared to herbaceous plants.

Thus, a need exists for an efficient photocatalytic material thatabsorbs electromagnetic energy efficiently for wavelengths below about450 nm. Additional requirements for an optimized photocatalyticcrop-protecting and yield-enhancing agent exist, including the cost andabundance of raw materials, ease of synthesis and application, andespecially a low environmental toxicity and thus well established safetyfor any materials comprising the agent.

SUMMARY

The present disclosure relates to photocatalytic compositions comprisingdoped titanium dioxide (TiO₂) nanoparticles, which are useful in thetreatment and prevention of microbial diseases and infestations, morespecifically, microbial diseases and infestations in plants.

In one embodiment, the invention provides a photocatalytic compositioncomprising titanium dioxide (TiO₂) nanoparticles doped with zinc (Zn)and having a ratio of titanium dioxide to zinc from about 5 to about150.

The photocatalytic composition can further comprise silicon dioxide(SiO₂). The ratio of titanium dioxide to silicon dioxide is from about 1to about 500.

The titanium dioxide nanoparticles preferably have an average particlesize of from about 2 nm to about 20 nm.

The photocatalytic compositions absorb electromagnetic radiation in awavelength range from about 200 nm to about 500 nm, and the absorbanceof light of wavelengths longer than about 450 nm is less than 50% theabsorbance of light of wavelengths shorter than about 350 nm.

Further, the present invention provides for a method for preventing ortreating microbial diseases and infestations in plants comprisingapplying the photocatalytic compositions taught herein to the surface ofa plant. The present invention also provides for a method forcrop-protecting and yield-enhancing of a plant comprising applying thephotocatalytic compositions taught herein to the surface of the plant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic representation of solar energy capture of variousTiO₂ compositions.

FIG. 2 is a graphic representation of the photocatalytic activity ofvarious TiO₂ compositions when irradiated at 354 nm.

FIG. 3 shows photocatalytic killing of Xanthomonas perforans on surfacestreated with various TiO₂ compositions using UV-A light.

FIG. 4 shows the effectiveness of various TiO₂ compositions inpreventing/reducing the number of leaf spot lesions per plant insunlight.

FIG. 5 shows the effectiveness of selected treatments for the control ofolive knot in sunlight.

FIG. 6 shows the effect of various TiO₂ compositions on conidialdevelopment of Sphaerotheca fuliginea/Erysiphe cichoracearum, the fungalcausal agent of powdery mildew, under sunlight.

DETAILED DESCRIPTION

The invention provides modified photocatalytic compositions that fulfillthe requirement for a broadly useful photocatalytic product for use onplants, and demonstrate superiority over unmodified nanoscale TiO₂.Further, the appropriate application rates have been evaluated. Thecompositions prevent black leaf spot on tomato plants, increase theyield of marketable fruit, reduce powdery mildew conidial formation oncantaloupe, and protect olive plants from microbially-induced tumors.The compositions contain only well characterized and safe materials, andcan be easily applied in the field using ordinary spray equipment. Theimprovements embodied in this invention afford the benefits ofphotocatalytic activity in settings of low UV irradiance, includinginterior artificial lighting.

The present invention relates to a photocatalytic composition comprisingzinc (Zn) doped titanium dioxide (TiO₂) nanoparticles, which is usefulin the treatment and prevention of microbial diseases and infestations,more specifically, microbial diseases in plants.

In one embodiment, the invention provides a photocatalytic compositioncomprising titanium dioxide (TiO₂) nanoparticles doped with zinc (Zn)having a ratio of titanium dioxide to zinc from about 5 to about 150.The ratio of titanium dioxide to zinc is preferably from about 40 toabout 100.

The photocatalytic composition can further comprise silicon dioxide(SiO₂). The ratio of titanium dioxide to silicon dioxide is from about 1to about 500, preferably from about 3 to about 20.

The titanium dioxide nanoparticles preferably have an average particlesize of from about 2 nm to about 20 nm.

A particularly preferred embodiment of the present invention provides aphotocatalytic composition which comprises:

(A) about 5000 to about 8000 ppm of titanium dioxide,

(B) about 50 to about 100 ppm of zinc, and

(C) about 500 to about 1000 ppm of silicon dioxide.

The photocatalytic composition absorbs electromagnetic radiation in awavelength range from about 200 nm to about 500 nm, and the absorbanceof light of wavelengths longer than about 450 nm is less than 50% theabsorbance of light of wavelengths shorter than about 350 nm.

Another embodiment of the present invention provides for a method fortreating or preventing microbial diseases and infestations in a plantcomprising applying a photocatalytic composition comprising titaniumdioxide (TiO₂) nanoparticles doped with zinc (Zn) having a ratio oftitanium dioxide to zinc from about 5 to about 150, to the surface of aplant.

Examples of plants to be treated include, but are not limited to, cropplants, which includes herbaceous and woody crop plants, for example,tomato plants, cucumber plants, citrus plants, olive and other drupeplants, apple and other pome plants, nut plants, and ornamental plants.

Examples of microbial diseases include, but are not limited to, leafspot disease, olive knot, fire blight, walnut blight, cherry canker, andpowdery mildew.

The present invention also provides for a method for increasing cropyield of a plant comprising applying a photocatalytic compositioncomprising titanium dioxide (TiO₂) nanoparticles doped with zinc (Zn)having a ratio of titanium dioxide to zinc from about 5 to about 150, tothe surface of a plant.

The present invention also provides for a method for treating orpreventing microbial disease or infestation on a surface comprisingapplying a photocatalytic composition comprising titanium dioxide (TiO₂)nanoparticles doped with zinc (Zn) having a ratio of titanium dioxide tozinc from about 5 to about 150, to a surface illuminated by artificiallight. The use herein of “surface” means an inanimate or an animateobject including plants.

Further, the invention provides for a method for treating or preventingmicrobial diseases or infestations in a plant comprising applying aphotocatalytic composition comprising titanium dioxide (TiO₂)nanoparticles doped with at least one doping agent, wherein the additionof the doping agent increases the absorbance of light across the rangeof about 200 nm to about 500 nm, and wherein the absorbance of light ofwavelengths longer than about 450 nm is less than 50% the absorbance oflight of wavelengths shorter than about 350 nm, to the surface of aplant. Preferably, the addition of the doping agent increases theabsorbance of light across the range of about 350 nm to about 450 nm.The doping agent useful in the photocatalytic composition is selectedfrom the group consisting of Ag, Zn, Si, C, N, S, Fe, Mo, Ru, Cu, Os,Re, Rh, Sn, Pt, Li, Na, and K, and combinations thereof. Particularlypreferred doping agents are Zn, Si, and Ag.

Further, the invention provides for a photocatalytic composition whichabsorbs electromagnetic radiation in a wavelength range from about 200nm to about 500 nm, and the absorbance of light of wavelengths longerthan about 450 nm is less than 50% the absorbance of light ofwavelengths shorter than about 350 nm. The composition comprisestitanium dioxide nanoparticles doped by at least one doping agent,wherein the doping agent disrupts the crystal lattice structure of thetitanium dioxide nanoparticles thereby altering the absorbance spectrumof the composition.

The invention provides photocatalytic materials that absorb an increasedproportion of available electromagnetic energy in a wavelength rangethat is selected to not substantially interfere with photosynthesis.However, it will be appreciated that the utility of the presentinvention is not limited to agricultural uses, since improvedutilization of the energy of light of wavelengths below 500 nm canafford benefit in a variety of settings. The invention is not limited toany particular theory or mechanism of photocatalytic benefit, sincephotocatalysis may provide benefit by multiple mechanisms, and we do notlimit the invention to a particular composition or type ofphotocatalyst. Also, the synthetic methods used to manufacture suchmaterials may be varied, and we do not limit the invention as to aparticular mode of manufacture.

Further, while the examples given here are based on TiO₂, a variety ofother photocatalysts such as Fe₂O₃ also may be similarly optimized, forexample by inclusion of SnO₂ at differing levels, and are contemplatedin this invention. The present invention is illustrated by the use offormulations of the invention dispersed in water for convenientapplication to a wide range of surfaces, but preparations contemplatedin the invention also may be dispersed in other solvents, and also mayutilize colorants, dispersants, carriers, and amphipathic agents tofacilitate ease of use or uniform application in selected settings.

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients are to be understood asbeing modified in all instances by the term “about”.

“At least one” as used herein means one or more and thus includesindividual components as well as mixtures/combinations.

The term “comprising” (and its grammatical variations) as used herein isused in the inclusive sense of “having” or “including” and not in theexclusive sense of “consisting only of.”

The terms “a” and “the” as used herein are understood to encompass theplural as well as the singular.

The terms “doped” or “doping” as used herein are understood to encompassthe introduction of one or more impurities (e.g., dopant, doping agent)into a material for the purpose of modifying the properties of thematerial.

The terms “treatment” and “treating” include mitigation of apre-existing microbial disease or infestation.

The terms “prevention” and “prophylaxis” include reduction of theincidence or severity of disease or infestation in either individuals orpopulations.

The invention will be further understood by the following examples,which are intended to be illustrative of the invention, but not limitingthereof.

EXAMPLES Example 1

Absorption characteristics of nanoscale TiO₂ were compared to nanoscaleTiO₂ doped with two differing zinc levels and SiO₂, over the wavelengthrange of 350 nm to 500 nm. The nanoparticle compositions weremanufactured by a modified sol-gel process, to produce formulationscontaining nanoparticles of anatase TiO₂ whose average size was 6 to 7nm. Zinc was incorporated as a doping agent to provide either low zinccontent (0.125% relative to TiO₂) or high zinc content (1.25% relativeto TiO₂). When SiO₂ was an additional dopant, it was present at 10%relative to TiO₂. The preparations were dried and absorbance wasmeasured using standard methods for obtaining diffuse reflectancespectra (DRS) of powders. The solar irradiance (hemispherical, 37 degreetilt) from ASTM G173-03 across this spectral range is shown forreference, (See FIG. 1).

It is evident upon inspection that the TiO₂ preparations doped withhetero-atoms absorb more strongly than otherwise similar undoped TiO₂ inthe near-UV and violet region of the spectrum. The doped preparationsabsorb 25 to 35 percent more of the energy available from 400 to 450 nm,a region where solar irradiance is relatively high but still outside themain photosynthetic action spectrum of plants.

Example 2 Photocatalytic Activity of Various Formulations of TiO₂ Dopedwith Zn and SiO₂ Under UV Illumination

The four formulations described in Example 1 were tested for theirphotocatalytic activity in a standardized system. Each preparation wassuspended in water at approximately 8000 ppm and applied to a glasspanel using a robotic high volume low pressure sprayer, and allowed todry for 24 hours. These panels were each attached to a glass tube toform a container, into which was placed 30 ml of an aqueous solution ofmethylene blue at a concentration providing an optical density of 2.3 at664 nm. The tubes were covered with a glass panel and subjected toillumination at an energy density of approximately 0.5 mW/cm² from alamp (GE item F18T8/BLB) affording ultraviolet illumination at 354 nm.This lamp provides no light at wavelengths below 300 nm or above 400 nm.The optical density of the methylene blue solution in each sample wasmonitored over a period of 48 hours and is shown in FIG. 2.

FIG. 2 shows that the nanocoatings caused a decline in optical density,which results from photocatalytic degradation of the organic dyemethylene blue. The coatings that had the higher amounts of dopantsafforded the most rapid declines, consistent with greater absorbance oflight from the lamp in the UV range (354 nm).

Example 3 Photocatalytic Activity of Various Formulations of TiO₂ Dopedwith Zn and SiO₂ Under Visible Light Illumination

The four formulations described in Example 1 were tested for theirphotocatalytic activity in a second system, in which the experimentalillumination was changed to more closely mimic relevant illuminationsuch as daylight or interior light, which are deficient in theultraviolet energy used in Example 2. Also, for this example thenanoparticle formulations were evaluated as colloidal suspensions in 20mM phosphate buffer, pH 7.2, rather than on a static surface. Theexperiment was performed in a 96-well plate format, in which each wellcontained methylene blue (observed OD₆₅₅ ranging from 0.05 to 0.5) and ananoparticle formulation or appropriate controls in a final volume of200 microliters. The plate was illuminated from a distance of 20 cm withlight from two Sylvania Gro-Lux lamps (F20 T12 GRO/AQ). These lamps emitonly 2% of their total emitted energy below 400 nm, whereasapproximately 36% of their total energy is emitted between 380 and 500nm, with a peak at 436 nm (reference: Technical Information Bulletin“Spectral Power Distributions of Sylvania Fluorescent Lamps”, OsramSylvania, www.sylvania.com).

The compositions of the four preparations tested in this experiment wereindependently verified by the analytical technique known as ICP-AES(inductively coupled plasma atomic emission spectrometry), whichdemonstrated their equivalent TiO₂ content and variations in Si and Zncomposition as described in Example 1. The nanoparticle preparationswere diluted in buffer to provide final concentrations of 75 ppm oftitanium dioxide of each formulation, with twenty replicate wells ofeach formulation. After a short period of equilibration in the dark,each plate was exposed to illumination with shaking, and optical densityat 655 nm was measured at multiple times over using a Molecular DevicesSpectraMax Plus spectrophotometer. The observed linear declines inoptical density due to each formulation were measured to give the ratessummarized in Table 1:

TABLE 1 Trial 1 Trial 2 TiO₂, low Zn 0.0017* 0.0016 TiO₂, low Zn, highSi 0.0020 Not tested TiO₂, high Zn, high Si 0.0019 Not tested TiO₂ onlyNot tested 0.0013 *All values reported are the decline in opticaldensity at 655 nm, per minute

It is evident that all the doped TiO₂ formulations show significantlyincreased rates (25% to 50%) compared to the undoped TiO₂ formulation.The magnitude of the increase in the rate of photocatalytic activity ishighly consistent with the increased absorption of light energy in therange of 400 nm to 450 nm that is evident in the spectra described inExample 1.

Example 4 Photocatalytic Killing of the Plant Pathogen XanthomonasPerforans on a Surface Using Incandescent Light

Sterile glass cover slips were separately coated with 0.5 ml volumes ofone of several types of nanoparticle suspensions (TiO₂, TiO₂/Ag orTiO₂/Zn). The nanoparticle compositions, comparable to those in Example2, were manufactured by a modified sol-gel process, to produceformulations containing nanoparticles of anatase TiO₂ whose average sizewas 6 to 7 nm, and which were doped with either Ag or Zn, using a ratioof TiO₂ to dopant of approximately 400:1 and approximately 800:1respectively. The coverslips were dried under sterile conditions. Astandardized inoculum of 0.1 ml of water containing 10⁷ copper-resistantXanthomonas perforans was applied to treated and untreated coverslips.The coverslips were then either illuminated with incandescent light atan illumination density of 3×10⁴ lux or maintained in a darkenvironment. At intervals, coverslips were placed in sterile centrifugetubes containing 10 ml of sterile water and vortexed. The recoveredbacteria were collected by centrifugation (14000×g, 3 minutes) andsuspended in 1 ml of sterile water. The numbers of viable bacteria inthe resulting suspensions were enumerated by standard plate dilutionmethods. The results are shown in FIG. 3.

Examination of FIG. 3 shows that the nanoparticle treatments resulted ina time-dependent and light-dependent killing of bacteria that is notobserved on the untreated coverslips. The rate of killing was faster forthe doped preparations than for undoped TiO₂. Of interest is the absenceof bacterial killing by non-illuminated TiO₂/Zn and TiO₂, whereas theTiO₂/Ag showed some killing of bacteria even when not illuminated,illustrating both the greater native toxicity of the material containingAg, and the requirement for illumination to provide energy for thephotocatalytic anti-bacterial effect.

Example 5 Infection of Tomato Plants by Xanthomonas Perforans, aCausative Agent of Leaf Spot, is Reduced by Treatment withPhotocatalytic Materials

Many bacterial diseases of plants are caused by uncontrolled expansionsof pre-existing populations of bacteria, which in low numbers do notcause disease. Thus, a major method to control these diseases inagriculture is to reduce the population of viable bacteria in order topreclude excessive expansion of bacteria that lead to damage and diseaseof the plant. Bacterial leaf spot of tomato is a disease system wheresuch a preventative approach is commonly sought.

A seed lot of tomato cultivar BHN 602 was naturally infected withXanthomonas perforans strain Xp1-7. The infected plants were treated atthe 3-4 leaf stage with nanoparticles (TiO₂, TiO₂/Ag & TiO₂/Zn) eitherundiluted or after tenfold dilution. The nanoparticle compositions,comparable to those in Example 4, were manufactured by a modifiedsol-gel process, to produce formulations containing nanoparticles ofanatase TiO₂ whose average size was 6 to 7 nm, and which were doped witheither Ag or Zn, using a ratio of TiO₂ to dopant of approximately 400:1and approximately 800:1 respectively. The nanoparticles were suspendedin water at concentration of 7,500-10,000 ppm or 5,000 to 8,000 ppm asindicated in FIG. 4. The plants were irrigated daily to keep the soilmoisture level at 85-95%, and misted with water two times a day for 15minutes each to enhance pathogen growth. Three plants were tested foreach treatment and the trial was set-up in a randomized complete blockdesign. Bacterial spot lesions were recorded before and two weeks afterthe treatment. Results are shown in FIG. 4. The error bar represents thestandard error of the mean.

It is evident that all nanoparticle treatments reduced the number ofbacterial spot lesions. The effectiveness of each preparation was notsignificantly affected by a ten-fold dilution in this experiment.Notably, the addition of a doping agent to the TiO₂ nanoparticlesimproved effectiveness compared to undoped TiO₂, consistent withincreased photocatalytic activity.

Example 6 Protection from Olive Knot Caused by Pseudomonas syringae Pv.savastonoi

Olive knot is a disease of olive trees caused by P. syringae pvsavastonoi, a motile gram negative bacterium that creates tumors (knots)in olive trees. The organism survives in these knots and is dispersedduring wet periods, whereupon it enters new sites via wounds includingleaf and flower abscission scars and those induced by mechanical injuryfrom wind, pruning, or frost. These knots inhibit proper plant growthand reduce fruit production. As in many other bacterial diseases ofplants, a reduction in the population of bacteria before disease isevident prevents or reduces the occurrence of olive knot, and methods toreduce the bacterial population are thus a common approach inagriculture.

In a greenhouse study, leaf scar wounds were inoculated with either 10⁵or 10⁸ P. syringae bacteria, and then sprayed a with a fifty-folddilution of the TiO₂/low Zn preparation described in Example 1, thusproviding a 250 ppm aqueous suspension, using a hand sprayer. Severalother agents were tested as controls. These control agents includeVantocil B (a combination of poly(hexamethylenebiguanideHCl withalkyldimethyl ammonium chloride), from Arch Chemicals, Inc, now part ofLonza Group Ltd, Basel Switzerland); Deccosan 321 (a mixture of severalquaternary ammonium salts, from Decco Cerrexagri Inc, Monrovia Calif.USA); Kasumin (kasugamycinHCl, from Arysta Lifescience N.A. LLC, CaryN.C. USA); Citrox (a proprietary mixture of citrus oil, detergent, andhydrogen peroxide, manufactured by Misco Products Corporation, ReadingPa., USA); and Kocide 3000 (copper hydroxide, from DuPont CropProtection, USA). The inoculated sites were wrapped with a single layerof Parafilm for one day to maintain enough moisture to ensure high ratesof infection, even though this reduced the amount of light at theinoculation site.

The first evidence for knot formation was observed after one month, anda quantitative first evaluation was performed after seven weeks. At thelower challenge inoculum, the nanoparticle TiO₂/low Zn treatment (termedAgriTitan in FIG. 5) was completely effective, similar to most othertested agents (FIG. 5). At the higher challenge inoculum, the spraytreatment with 250 ppm TiO₂/low Zn continued to be fully effective,similar to the current standard treatment of 1000 ppm copper hydroxide.All other tested agents were less effective (FIG. 5).

Example 7 Tomato Field Experiment

The TiO₂ preparation doped with Zn used in the greenhouse experiment wasselected for use in a field trial. Zn was selected as the dopant forfurther investigation due to its approval by the U.S. EnvironmentalProtection Agency as a minimal risk pesticide, a status not accordedother potential doping agents. Field trials were performed to comparethe effectiveness of TiO₂ doped with Zinc at a ratio of 800:1(formulated as a 0.7% colloidal suspension in H₂O) to standardtreatments for prevention or control of leaf spot on tomato plants. Eachtreatment group contained 48 plants (12 per plot, 4 replicates), and thetrial used a randomized complete block design. The TiO₂/Zn was dilutedin water to provide a range of application rates. Controls included acopper sulfate formulation either alone or in conjunction with manzate,and no treatment.

The plants were sprayed with test materials at weekly intervals (8times) starting from the first week after transplanting. Diseaseseverity was rated at monthly intervals using a non-dimensional 12-pointscale, to assess the percentage of canopy affected by bacterial leafspot (Horsfall et al., Phytopathology, vol. 35, 655, Abstract, 1945).These values were converted to mid-percentages and used to generate anArea Under Disease Progression Curve (AUDPC). Marketable yield databased on USDA grades also were taken from the field trial to determinewhether the nanoscale formulations had any herbicidal action on tomatoplants. The results are shown in Table 2 and Table 3.

TABLE 2 Effect of TiO_(2/)Zn on the incidence of bacterial spot ontomato (variety BHN 602), shown as average area under the diseaseprogress curve (AUDPC). Treatment Dilution AUDPC^(y) TiO_(2/)Zn X/10^(x) 800.6 c^(z) X/20  950.3 bc X/40 1000.1 b  X/60 1033.4 ab X/80  933.6 bcX/100 1050.0 ab Copper 1050.0 ab Copper + Manzate 1033.4 ab Untreated1181.3 a  ^(x)X represents the undiluted formulation of TiO_(2/)Zn.^(y)Disease severities were rated using the Horsfall-Barratt scale, anon-dimensional 12-point scale, to assess the percentage of canopyaffected by bacterial spot. Values were converted to mid-percentages andused to generate AUDPC. ^(z)Column means indicated with the same lettersare not significantly different (P ≦ 0.05) based on Student Newman Keulstest.

The results of the field trial summarized in Table 2 show that TiO₂/Znprovided better protection against spontaneous leaf spot disease thaneither of the conventional treatments. In pair-wise comparisons, the1:10 diluted TiO₂/Zn was statistically significantly better than eitherof the control treatments, demonstrating more than a 20% reduction inthe AUDPC. Also, a comparison of the results for all dilutions ofTiO₂/Zn as a group to the controls as a group showed statisticalsignificance (p<0.05).

TABLE 3 Effect of TiO_(2/)Zn on the yield of tomato (kg/ha). Yield(kg/ha) ^(y) Extra Total Treatment Dilution Medium Large LargeMarketable TiO_(2/)Zn X/10 ^(x)  10047 a ^(z) 18581 a 38563 a 67191 aX/20 8702 a  16070 ab 38622 a 63394 a X/40 9248 a 18779 a 32594 a 60620a X/60 8397 a  16980 ab 40102 a 65478 a X/80 6393 a  16392 ab 32446 a55231 a X/100 8457 a 18319 a 35091 a 61867 a Copper 7744 a 13642 b 32006a 53392 a Copper + 7653 a 14549 b 35656 a 57859 a Manzate Untreated 6312a  15293 ab 32670 a 54364 a ^(x) X represents the undiluted formulationof TiO_(2/)Zn. ^(z) Column means indicated with the same letters are notsignificantly different (P ≦ 0.05) based on Student Newman Keuls test.

The results of the field trial summarized in Table 3 show that TiO₂/Zndid not adversely affect the yield of marketable tomato. In fact,increased yields were observed for the TiO₂/Zn treated groups; the totalmarketable yield from the plants treated with the tenfold dilutedTiO₂/Zn material was more than 20% greater than the yield from anycontrol. Although this difference was not statistically significant inindividual pair-wise comparisons owing to variations in yield within ineach group, a statistical test comparing the “total marketable” resultsfor the six dilutions of TiO₂/Zn as a group to the three controltreatments as a group was statistically significant (p<0.05).

Example 8

A replicate of Example 7 was performed in the next growing season. Theprotocol was identical to Example 7, wherein various dilutions of thenanoscale TiO₂/low Zn aqueous preparation were applied weekly byconventional high volume, low pressure compressed air spray to tomatoesin the field in a random block design, with appropriate controls. Theresults for disease progression are presented in Table 4, below, anddemonstrate a concentration dependent control of disease. Unfortunately,yield data are not available for this experiment due to severe damagefrom a hailstorm prior to fruit harvest.

TABLE 4 Effect of TiO₂/Zn on the incidence of bacterial spot on tomatocultivar ‘BHN 602’, shown as average area under the disease progresscurve (AUDPC). Treatment Dilution^(x) AUDPC^(y) TiO₂/Zn X/10 583.6d^(z ) X/20  741.1 abc X/40 724.5 bc X/60  752.5 abc X/80  757.8 abcX/100 806.8 ab Kocide 3000 864.5 ab Kocide3000 + Penncozeb 75DF 642.3 cdUntreated 892.9 a  LSD 107.5   P > F  <0.0001 ^(x)X represents theundiluted formulation of TiO₂/Zn. ^(y)Disease severities were ratedusing the Horsfall-Barratt scale, a non-dimensional 12-point scale, toassess the percentage of canopy affected by bacterial spot. Values wereconverted to mid-percentages and used to generate AUDPC. ^(z)Columnmeans indicated with the same letters are not significantly different (P≦ 0.05) based on Student Newman Keuls test. Yield data could not betaken due to significant hail damage on tomatoes one week before thefirst harvest date.

Example 9

A third field trial in the tomato spot disease system was conducted inthe next growing season. Based on the results of Example 7 and Example8, we did not test the more dilute application rates of TiO₂/Zn, andthus the ability to detect a clear relationship to the applied rates wasreduced. However, the general result remained the same (Table 5).

TABLE 5 Effect of TiO₂/Zn on the incidence of bacterial spot on tomatocultivar ‘BHN 602’, shown as average area under the disease progresscurve (AUDPC), and the yield of tomato fruit. Yield (kg/ha) Extra TotalTreatment Dilution^(x) Medium Large Large Marketable AUDPC^(y) TiO₂/ZnX/10 4,518 a^(z) 7,512 a 15,251 a 27,281 a   995.8 c X/20 4,966 a 6,994a 11,200 a 23,161 a 1,073.6 bc X/40 4,210 a 7,900 a 17,409 a 29,519 a1,089.4 bc Kocide 3000 4,532 a 6,986 a 11,258 a 22,776 a 1,306.4 abKocide 3000 + 3,909 a 8,202 a 14,933 a 27,044 a   816.4 c Penncozeb 75DF Untreated 5,449 a 7,093 15,159 a 27,700 a 1,410.5 a LSD ns ns ns ns  277.3 P > F    0.7194    0.9652    0.4146    0.7845    0.0037 ^(x)Xrepresents the undiluted formulation of TiO₂/Zn. ^(y)Disease severitieswere rated using the Horsfall-Barratt scale, a non-dimensional 12-pointscale, to assess the percentage of canopy affected by bacterial spot.Values were converted to mid-percentages and used to generate AUDPC.^(z)Column means indicated with the same letters are not significantlydifferent (P ≦ 0.05) based on Student Newman Keuls test.

Thus, in this third field trial, the application of nanoparticles ofTiO₂/low Zn again demonstrated activity in controlling bacterial spot oftomato, on the basis of disease severity and fruit yield. At a dilutionof 1:10, TiO₂/Zn was statistically superior to either single agentcopper or untreated controls.

Example 10 The Effect of TiO₂/Zn, Formulated as in Examples 7 and 8, onConidial Development of Sphaerotheca fuliginea/Erysiphe cichoracearum,the Fungal Causal Agent of Powdery Mildew

Cucumber plants with an approximately equal number of lesions on theleaves were tagged for the experiment in greenhouse conditions. Threeleaves were used for each treatment. In FIG. 6, “n” represents theaverage number of lesions on these leaves. The lesions were in the range(0.1-0.6 cm). The plants were sprayed with TiO₂/Zn of formula 1 at 1/50and 1/100× concentrations using a hand sprayer. Untreated plants weresprayed with sterile distilled water. The plants were kept in thegreenhouse for 48 hours. Leaves were removed from the plants and lesionswere observed under a microscope for presence or absence of conidia.(See FIG. 6).

These results show that application of the photocatalytic preparationsignificantly reduced the ability of powdery mildew to produce conidia,which are essential to its reproduction.

It is important to note that the construction and arrangement of themethods and steps shown in the exemplary embodiments is illustrativeonly. Although only a few embodiments of the present disclosure havebeen described in detail, those skilled in the art will readilyappreciate that many modifications are possible without materiallydeparting from the novel teachings and advantages of the subject matterrecited in the claims. Accordingly, all such modifications are intendedto be included within the scope of the present disclosure as defined inthe appended claims. The order or sequence of any process or methodsteps may be varied or re-sequenced according to alternativeembodiments. Other substitution, modification, changes and omissions maybe made in the design, operating conditions and arrangement of theembodiments without departing from the spirit of the present disclosureas expressed in the appended claims.

All publications, patents and patent applications cited in thisspecification are herein incorporated by reference, and for any and allpurposes, as if each individual publication, patent or patentapplication were specifically and individually indicated to beincorporated by reference. In this case of inconsistencies, the presentdisclosure will prevail.

The invention claimed is:
 1. A method for treating or preventingmicrobial diseases and infestations in a plant consisting of: applying aphotocatalytic composition to a surface of the plant, the photocatalyticcomposition consisting essentially of titanium dioxide (TiO₂)nanoparticles doped with at least one doping agent, wherein the additionof the doping agent increases the absorbance of light across the rangeof about 200 nm to about 500 nm, and wherein the absorbance of light ofwavelengths longer than about 450 nm is less than 50% the absorbance oflight of wavelengths shorter than about 350 nm.
 2. The method of claim1, wherein the addition of the doping agent increases the absorbance oflight across the range of about 350 nm to about 450 nm.
 3. The method ofclaim 1, wherein the at least one doping agent is selected from thegroup consisting of Ag, Zn, Si, C, N, S, Fe, Mo, Ru, Cu, Os, Re, Rh, Sn,Pt, Li, Na, and K.
 4. The method of claim 1, wherein the at least onedoping agent is Zn, Si, or a combination thereof.
 5. The method of claim1, wherein the at least one doping agent is Zn.